Typically, an electrodeless discharge lamp is comprised of a lamp unit, a power coupler unit (inductive coil device), and so on, and excites a discharge gas contained in the bulb (discharge container) by applying a high-frequency electromagnetic field to the discharge gas, and thereby emitting light. Since no electrode is provided in a bulb of the electrodeless discharge lamp, it has a long life as compared to a discharge lamp having an electrode in the bulb. Further, by appropriately selecting the type and pressure of the discharge gas to be contained in the bulb, the strength of the high-frequency magnetic field, and so on, a high-efficiency electrodeless discharge lamp can be achieved.
Due to the high efficiency and the long life, an electrodeless discharge lamp is especially advantageous in the case where it is used at a place that requires a high efficiency as well as making it difficult to replace the lamp unit such as for illumination at the ceiling of a theater or an entrance hall or for illumination at a road.
For example, FIG. 14 shows a configuration of a conventional electrodeless discharge lamp disclosed in International Publication No. WO97/40512 or Japanese Laid-open Patent Publication No. 2004-119038. This conventional electrodeless discharge lamp is comprised of a lamp unit 1 and a power coupler unit 2, in which a coil is wound in a substantially cylindrical cavity (hollow portion) 11 formed at the center of the lamp unit 1. The lamp unit 0 comprises a bulb (airtight container) 10 having the above described cavity 11, a coupling member (collar) 30 that is made of a synthetic resin for fixing the bulb 10 to the power coupler unit 2, and so on. For example, an ionized enclosure having a rare gas is contained in the bulb 10. On the other hand, the power coupler unit 2 comprises the coil 20, a soft magnetic core 21, a heat conduction member 22, a mounting member 31 made of a synthetic resin to be coupled with the coupling member 30, and so on.
The coupling member 30 and the mounting member 31 are precisely formed of a synthetic resin and have a number of complex shaped fitting portions 31a, 31b, 31c, 31d, 31e . . . . These fitting portions 31a, 31b, 31c, 31d, 31e . . . allow the lamp unit 1 to be attached on and detached from the power coupler unit 2 as well as allowing the lamp unit 1 to be securely held so as not to become easily detached from the power coupler unit 2 while the lamp unit 1 is mounted on the power coupler unit 2.
However, since the coupling member 30 and the mounting member 31 are made of the synthetic resin as described above, they may deteriorate gradually during long-term use depending on environmental conditions and thus may have looseness, deformation, wear, defect, and so on at the respective fitting portions. Especially when it is used, for example, in a high-temperature environment, at a place with much ultraviolet radiation from the lamp unit itself or sunlight, or at a place where heavy vibrations may occur such as on a road or an iron bridge, the coupling member 30 and the mounting member 31 may seriously deteriorate. Consequently, in such a case where the electrodeless discharge lamp is used on a ceiling for example, there is a possibility that the lamp unit 1 may be detached from the power coupler unit 2 because the coupling structure between the coupling member 30 and the mounting member 31 cannot maintain the coupling power sufficient for the weight of the lamp unit 1. Therefore, it is needed to prevent the lamp unit 1 from accidental detachment from the power coupler unit 2 in long-term use.
Further, since the electrodeless discharge lamp is often used at a place where replacement of the lamp unit is difficult, the workability in mounting is particularly important. Therefore, the lamp unit is required to be easily replaceable even when a worker performs the replacement by touch. Since the conventional electrodeless discharge lamp has a number of intricately shaped fitting portions 31a, 31b, 31c, 31d, 31e . . . on the coupling member 30 and the mounting member 31, it is not always easy to replace the lamp unit 1.